1. Field of the Invention
The present invention relates to a valve operating device for use in an internal combustion engine for varying operating characteristics of an engine valve depending on an operating condition of the engine.
2. Description of the Prior Art
One conventional valve operating device is known from Japanese Patent Application Laid-open No. 57805/88, for example.
In the valve operating device disclosed in the above patent application, three rocker arms are disposed in an adjacent arrangement, and the adjacent rocker arms in two sets are connected to and disconnected from each other. This allows a pair of engine valves connected to the outer rocker arms to be varied in their operating characteristics depending upon three operating conditions: a low-speed operating condition, a medium-speed operating condition and a high-speed operating condition of the engine. In the prior art device, however, a selective coupling means for connecting the adjacent rocker arms in one of the sets to each other and a selective coupling means for connecting the adjacent rocker arms in the other set to each other are disposed coaxially and hence, in order to insure an operational stroke of each selective coupling means, the width of the three adjacent rocker arms along their swinging axes must be set larger. Therefore, the valve-operating device has an increased size, and in order to insure the rigidity at the time when the rocker arms are connected, it is necessary to increase the strength of the rocker arms.
There is also a conventionally known valve operating device including a trigger mechanism mounted to the selective coupling means for restraining the operational timing for the selective coupling means, as disclosed in Japanese Patent Application Laid-open No. 31611/86. If the trigger mechanism is applied to a valve operating device of the conventional type described above, which includes a pair of selective coupling means disposed coaxially, however, the axial length of a connecting piston as a component of the selective coupling means should be set relatively long. For this reason, the width of the rocker arms along their swinging axes must be further increased. Further, there is a conventionally known valve operating device which includes rocker arms each movable to follow a cam through a roller follower in order to reduce the resistance of friction with the cam, and selective coupling means disposed coaxially with the roller follower, as disclosed in Japanese Patent Publication No. 50286/90. In such a conventional valve operating device including a pair of the selective coupling means disposed coaxially, however, if the means are disposed coaxially with the roller followers, the width of the rocker arms must be further increased in order to insure a space occupied by the roller followers.
On the other hand, in the valve operating device disclosed in the above Japanese Patent Application Laid-open No. 57805/88, the operating characteristics of each of the engine valves is varied at two stages of a disconnection condition in which the engine valve is opened and closed in accordance to the operation of the rocker arm connected to the engine valve, and a connection condition in which the engine valve is opened and closed in accordance to the operation of the adjacent rocker arm. Therefore, in order to achieve the variation of the operating characteristics at three stages according to the operating conditions of the engine, a combination of these stages must be selected from a limited combination of connection and disconnection of two sets of rocker arms. In order to enable a more precise varying control of valve-operating characteristics according to the operating conditions of the engine, it is desirable to increase the freedom of selection of a combination of the valve-operating characteristics in each operating region.